The present invention relates to semiconductor devices, and more particularly to a method of enhancing the uni-directional diffusion of a metal during silicidation using an initial anneal that includes two distinct thermal steps (hereinafter thermal cycles). The present invention also relates to semiconductor structures that include the metal silicide produced using the method of the present invention.
One type of material commonly employed in fabricating ohmic contacts for semiconductor devices is metal suicides such as cobalt silicide (CoSi) or nickel silicide (NiSi). Metal suicides are typically fabricated using a conventional self-aligned silicide (i.e., salicide) process. In such a process and when CoSi contacts are desired, a blanket TiN/Co film is deposited over exposed Si-containing regions (e.g., source, drain and gate). A first annealing step that is performed at a temperature from about 400° to about 600° C. is then employed to form a cobalt monosilicide. A selective etch is then employed to strip the TiN cap and to remove any unreacted Co that was not converted into the cobalt monosilicide film. The cobalt monosilicide is then subjected to a second anneal which converts (i.e., transforms) the cobalt monosilicide into cobalt disilicide. As is known to those skilled in the art, cobalt disilicide has a lower resistance than cobalt monosilicide.
Despite being able to form metal silicide contacts, the self-aligned silicide process mentioned above has several problems associated therewith. In particular, in the conventional silicide process mentioned above, both the metal, e.g., Co, and Si interdiffuse through a thin oxide. This bi-lateral diffusion is illustrated in FIG. 1 A, wherein reference numeral 10 is a Si-containing material, reference numeral 12 is a Co layer, reference numeral 14 designated by the broken line is a native oxide layer, reference numeral 16 shows the direction of Si diffusion, and reference numeral 18 shows the direction of Co diffusion. As is shown, the Si diffuses up, while the Co diffuses down in the conventional self-aligned silicide process.
After formation of cobalt disilicide using the conventional self-aligned silicide process, there is a small precipitate of SiO2 that forms in the cobalt disilicide film. This is shown, for example, in FIG. 1B wherein reference numeral 20 denotes the cobalt disilicide film and reference numeral 22 denotes the SiO2 precipitate.
The SiO2 precipitate 22 shown in FIG. 1B is originated from a native oxide that was present on the surface of the Si-containing material 10 prior to performing the self-aligned silicide process. The native oxide is difficult to remove even with the numerous surface cleaning methods that are presently available.
In addition to the above problem, the conventional self-aligned silicide process also creates Si-containing gate voiding issues, when the Si-containing gate width is below 70 nm. The voiding issue is shown in FIG. 1C. In the drawing, reference numeral 10 is a Si-containing substrate, reference 24 is a Si-containing gate, reference numeral 20 is a cobalt disilicide film formed atop the Si-containing gate 24, reference numeral 26 is the void in the Si-containing gate 24, and reference numeral 28 is an insulator spacer that is present on the sidewalls of the Si-containing gate 24. The voiding issues mentioned above and depicted in FIG. 1C cause the resistance of the Si-containing gate 24 to increase.
A recent improvement in the conventional self-aligned silicide process has been described in co-assigned U.S. Pat. No. 6,323,130 B1 to Brodsky, et al. Specifically, the process disclosed in Brodsky, et al. includes the steps of: forming a metal silicon alloy layer containing less than about 30 atomic % Si, the remainder is Co and/or Ni, over a silicon-containing substrate containing an electronic device to be electrically contacted, first annealing the metal silicon alloy layer at a temperature from about 300° to about 500° C. so as to form a metal rich silicide layer that is substantially non-etchable compared to the metal silicon alloy or pure metal, selectively removing any unreacted metal silicon alloy over non-silicon regions, and second annealing the metal rich silicide layer under conditions effective in forming a metal silicide phase that is in its lowest resistance phase. An optional oxygen diffusion barrier layer may be formed over the metal silicon alloy layer prior to the first annealing step.
The process described by Brodsky, et al. undergoes a different diffusion mechanism in the first annealing step than the bi-directional diffusion mechanism mentioned above for the conventional self-aligned silicide process. In particular, only Co diffuses downward forming a metal rich silicide layer in the process disclosed by Brodsky, et al. The resultant uni-directional mechanism achieved using the Brodsky, et al. process is illustrated in FIG. 2A. In this drawing, reference numeral 10 is a Si-containing material, reference 30 is the metal silicon alloy layer, reference numeral 14 is a native oxide layer, and reference numeral 18 shows the direction of Co diffusion.
As with the conventional self-aligned silicide process, the process disclosed in Brodsky, et al. removes any unreacted Co metal after the first annealing step. As indicated at Col. 5, lines 19–21 of Brodsky, et al., a mixture of hydrogen peroxide and sulfuric acid can be used as the etchant for removing the “residual” Co that was not transformed into a metal rich silicide phase. During this etching process, the Si in the Co metal oxidizes and forms a SiO2 surface layer on the metal rich silicide. The structure is shown, for example in FIG. 2B, wherein reference numeral 32 is the metal rich silicide phase and reference numeral 22 is the surface SiO2 layer.
Next, a second anneal is performed in the Brodsky, et al. process that transforms the metal rich silicide into cobalt disilicide. The second anneal is performed at a temperature from about 600° to about 900° C.
There are several advantages of the uni-directional diffusion mechanism achieved utilizing the process disclosed by Brodsky, et al. over the conventional self-aligned silicide process wherein bi-directional diffusion occurs. First, since the excess Co metal on top of the native oxide was etched away, there is no SiO2 precipitate in the resultant cobalt disilicide film, which leads to low sheet resistance. Second, uni-directional diffusion has only the Co diffusing into Si and no Si is diffusing out, therefore, there are no void issues with narrow Si-containing gates.
Despite the improvements achieved with the process disclosed in Brodsky, et al., the Brodsky, et al. process results in a Co diffusion depth that is self-limiting. For example, in the first annealing step, a cobalt rich silicide film of about 3 nm is formed. After the strip and with the second anneal, the 3 nm cobalt rich silicide is transformed into a 12 nm cobalt disilicide film. In normal complementary metal oxide semiconductor (CMOS) processing, the contact reactive ion etch and sputter cleaning steps typically remove from about 10 to about 12 nm of cobalt disilicide, which is right at the boundary of cobalt disilicide formation for the process disclosed by Brodsky, et al. When the cobalt disilicide film is completely removed, a contact resistant problem with the metal to diffusion contact is evident.
In most CMOS applications, it is preferred to have a 18–24 nm cobalt disilicide process window issue. In normal processing, to increase the diffusion thickness, one would increase the first annealing temperature used in the silicide process. However, in this case, the maximum temperature is limited to about 450° C. because (i) higher temperature annealing would result in the formation of cobalt disilicide from the cobalt silicide alloy at the trench isolation regions, which leads to leakage, i.e., bridging, between different active areas on a substrate, an active area and the Si-containing gate, and between different Si-containing gates on the same substrate, and (ii) higher temperature anneals in the first annealing step lead to bi-directional diffusion, which leads to the same problems mentioned above in the conventional self-aligned silicide process.
In view of the above, there still exists a need for providing an improved method of forming metal silicide contacts that avoids the problems associated with the conventional self-aligned silicide process and with the silicide process mentioned in Brodsky, et al.